The present invention relates to a method of reducing doming in a colour display tube and to a colour display tube made in accordance with the method.
Colour display tubes consist of an envelope comprising a faceplate, a cone and a neck. An electron gun system is arranged in the neck. A cathodoluminescent multicolour screen is provided on the interior of the faceplate and an apertured shadow mask is mounted at a short distance from the screen. In operation three electron beams produced by the electron gun system are scanned across the shadow mask by deflection coils mounted at the outside of the neck-cone transition of the envelope. A high percentage of the electrons, up to 80%, impinge upon the shadow mask causing it to heat-up. In the initial phase after switch-n the apertured portion of the shadow mask expands and is deflected or domes towards the screen. This causes mislanding of the electron beams leading to colour distortion. Subsequently heat is lost from the apertured portion of the shadow mask by radiation to the faceplate and by conduction to the frame of the shadow mask which expands and in so doing tightens the shadow mask sheet causing it to be deformed oppositely. This tightening of the shadow mask sheet together with a suitable temperature compensating shadow mask suspension system such as that disclosed in British Patent Specification No. 1192725 or British Patent Specification No. 2097996A ideally causes the shadow mask to be restored to its undeflected state.
U.S. Pat. No. 3,392,297 (Schwartz) discloses applying a layer of a heat absorptive material to the aluminium layer normally covering the phosphors of the cathodoluminescent screen. The patentee remarks that by the screen/faceplate absorbing radiated heat from the part-spherical shaped shadow mask a temperature equalisation state is achieved and consequently doming is compensated for.
U.S. Pat. No. 3,878,428 (Kuzminski et al) discloses applying one of a variety of heat absorbing layers to the centre portion of the screen and a heat reflective material to a peripheral portion of the forward facing surface of the shadow mask; the purpose of this mixture of layers again being to equalise more easily the temperature difference between the part spherical shadow mask and the screen.
In spite of these known methods to counter doming and the consequent misregistration of colours, there still exists a doming problem which is more pronounced in flatter, squarer display tubes currently entering production.
An object of the present invention is to improve the anti-doming characteristics of a colour display tube.